The present invention relates generally to an automatic diaphragm control mechanism for a single lens reflex camera and it relates more particularly to a mechanism for detecting scene brightness through the objective of a camera, that is, of the TTL light measuring type.
The automatic diaphragm control mechanism of the subject nature has been described, for example, in U.S. Pat. No. 3,777,637. In the known device, a photo-electric element is disposed at the top of a pentagonal prism to receive scene light through the objective of the camera, and the diaphragm aperture previously opened fully for viewing, is stopped down gradually by a diaphragm driving mechanism operable substantially in the same manner as performed by a conventional mechanism for driving the diaphragm to a pre-set value prior to shutter opening. Accordingly, the known mechanism can be so constructed as to accept ordinary interchangeable objective lenses adapted for the conventional diaphragm driving mechanism without requiring any particular additional structure for the automatic diaphragm control. However, the known mechanisms possess the disadvantages that the automatic diaphragm adjusting must be carried out before the reflex mirror of the camera moves from its viewing position, because the photo-electric element receives light via the reflex mirror. In other words, the known mechanisms determine the diaphragm aperture in accordance with scene light detected some time, but not immediately before the shutter actuation, because of the time consumed by the mirror operation. Further, in the known devices, the light measurement is frequently influenced by unnecessary stray light entering through the eyepiece.